The identification of dyes and organic colorants in works of art is one of the most difficult problems in the field of scientific studies of cultural heritage.
Organic colorants are, by definition and by economic necessity, molecules with high extinction coefficients in the visible radiation range. In other words, organic colorants provide intense coloration even when used in small quantities. The actual amount of organic dye present in a dye fiber or contained as an insoluble lake in a paint sample is extremely small. Only the most sensitive microanalytical, or better, non-invasive techniques can be used to identify an organic dye in works of art.
Various natural anthraquinone dyes, including alizarin, purpurin, carminic acid, and laccaic acids, such as shown in FIG. 1, are examples of important natural products that are extremely difficult if not impossible to analyze by dispersive Raman spectroscopy. Anthraquinones are mordant dyes: they are used not in their free form, but as insoluble complexes with aluminum or other metal ions. These dies are discussed in the following publications: Schweppe, H, and Winter, J., “Madder and Alizarin” in West Fitzhugh, E. (editor) Artists' pigments; a handbook of their history and characteristics, Volume 3, National Gallery of Art, Washington (1997) 109-142; and Schweppe, H., and Roosen-Runge, H., “Carmine—cochineal carmine and kermes carmine” in Feller, R. L. (editor) Artists' pigments, A handbook of their history and characteristics, Volume 1, National Gallery of Art, Washington (1986) 255-283, the disclosures of which are incorporated herein by reference. The same dyes are also found as pigments in works of art such as drawings, illuminated manuscripts, prints, and paintings, where they are found as “lake pigments”, that is, insoluble complexes of the dye molecule with a poly-valent metal cation. Used as writing inks, the lakes derived from these dyes and from synthetic organic dyes could also be found in written or printed documents.
Organic dyes in historic textiles currently are identified by High Performance Liquid Chromatography, which is discussed in Wouters, J. “High performance liquid chromatography of anthraquinones: analysis of plant and insect extracts and dyed textiles,” Studies in Conservation (1985) 30(3) 119-128, the disclosure of which is incorporated herein by reference. In practice, this process involves removing a sample of up to 5 mm of threads of the textile and carrying out hydrolysis/extraction separation with an acidic solution. Due to the required sample size, this process is ill-suited to identify the organic dyes in paintings and other works of art where sufficient samples cannot be obtained.
Natural dyes in works of art have been identified by the use of surface enhanced Raman scattering (hereinafter “SERS”) methods. Various publications that discuss surface enhanced Raman scattering include: Leona, M. IRUG Proceedings Volume of the Sixth Infrared and Raman Users Group Conference, Florence (I) 29.03-01.04.2004, Editor Marcello Picollo, Publisher II Prato (Padova, Italy) 2005, 105-112; Canamares M V, Garcia-Ramos J V, Domingo C, Sanchez-Cortes S. J. Raman Spectrosc. 2004; 35: 921-927; Shadi I T, Chowdry B Z, Snowden M J, Whitnall R. J. Raman Spectrosc. 2004; 35: 800-807, Leona M., Proceedings of SPIE Vol. 5993, 59930L-1, 2005, the disclosures of which are incorporated herein by reference.
SERS is a complex effect observed when organic molecules are adsorbed on atomically rough metal surfaces. See, for example, Fleischmann, M., Hendra, P. J., and McQuillan, A. J., “Raman spectra of pyridine adsorbed at a silver electrode,” Chemical Physic Letters (1974) 26(2) 163-166; Jeanmaire, D. L., and Van Duyne, R. P., “Surface Raman spectroelectrochemistry. Part 1: heterocyclic, aromatic, and aliphatic amines adsorbed on the anodized silver electrode,” Journal of Electroanalytical Chemistry (1977) 84(1) 1-20; Albrecht, M. G., and Creighton, J. A., “Anomalously intense Raman spectra of pyridine at a silver electrode,” Journal of the American Chemical Society (1977) 99 5215-5227; Nie, S., and Emory, S. E., “Probing single molecules and single nanoparticles by surface-enhanced Raman scattering,” Science (1997) 275 1102-1106; Kneipp, K., Wang, Y., Kneipp, H., Perelman, L. T., Itzkan, I., Dasari, R. R., and Feld, M. S., “Single molecule detection using surface-enhanced Raman scattering (SERS),” Physical Review Letters (1997) 78(9) 1667-1670; and Campion, A. and Kambhampati, P., “Surface-enhanced Raman scattering,” Chemical Society Reviews (1998) 27 241-250, the disclosures of which are incorporated herein by reference.
In short, when studying by Raman spectroscopy organic molecules adsorbed on atomically rough metal surfaces, the incident (laser) and scattered (Raman) electromagnetic fields are intensified by resonance with the metal plasmon (the surface electrons standing wave). Additionally, interactions between the energy levels of the metal and of the molecule can lead to resonance conditions or quench any fluorescence through ultrafast electron transfer from the molecule excited states to the metal, as discussed in Shoute, L. C., and Loppnow, G. R., “Excited state dynamics of alizarin-sensitized TiO2 nanoparticles from resonance Raman spectroscopy,” Journal of Chemical Physics (2002) 117(2) 842-850, the disclosure of which is incorporated herein by reference.
The observation of single molecule Raman spectra from species that are known to be fluorescent implies that SERS produces an enhancement of the Raman effect of up to 14 orders of magnitude. This enhancement has tremendous implications towards ultra-sensitive detection of natural dyes in works of art, the technique is extremely promising for analyzing microscopic samples of lake pigments and dyed fibers. While SERS is in theory possible with any nanosized metal support, in practice, plasmon resonance limits the metals which support SERS to those which show plasmon resonance at the wavelength commonly used for Raman excitation: these are most commonly gold, silver and copper. In the course of this study, silver was elected as the most effective support. The nanoscale roughness condition was satisfied by using solution-reduced colloids, a very common SERS support, prepared following the procedure described in Lee, P. C., and Meisel, D., “Adsorption and surface-enhanced Raman of dyes on silver and gold sols,” Journal of Physical Chemistry (1982) 86 3391-3395, the disclosure of which is incorporated herein by reference.
SERS has been proven to allow detection of alizarin, the main constituent of the historical dye madder (from the root of Rubia tinctorum L.) at picogram levels (in Leona, IRUG Proceedings, referenced above) and of other dyes (Leona, Proceedings of SPIE, referenced above). Current procedures for SERS of works of art however still require that a sample be removed from the work of art to hydrolyze the dye-mordant-fiber complex and bring the dye into solution. The dye solution is then mixed with the silver colloid and its Raman spectrum measured. FIG. 2 shows the SERS spectrum of alizarin in a silver colloid with poly-L-lysine (the lower line is the spectrum of an Ag and NaOH blank): poly-L-lysine is used to facilitate adsorption of the dye on the silver nanoparticles.
Other publications pertinent to the field include Tiedemann, E. J., and Yang, Y., “Fiber safe extraction of red mordant dyes from hair fibers,” Journal of the American Institute for Conservation (1995) 34(3) 195-206; Bell, S. E. J., and Spence, S. J., “Disposable, stable media for reproducible surface-enhanced Raman spectroscopy,” Analyst (2001) 126 (1), 1-3; and Farquharson, S., and Maksymiuk, P., “Simultaneous chemical separation and surface enhanced Raman spectral detection using silver doped sol-gels,” Applied Spectroscopy (2003) 57(4) 479-481, the disclosures of which are incorporated herein by reference.
Various patents related to the field include U.S. Pat. No. 7,022,288 entitled “Chemical Detection Sensor System”; U.S. Pat. No. 6,943,032 entitled “Chemical separation and plural point, surface enhanced Raman spectral detection using metal doped sol-gels”; U.S. Pat. No. 6,943,031 entitled “Simultaneous Chemical separation and surface enhanced Raman spectral detection using metal doped sol-gels”; U.S. Pat. No. 6,649,683 entitled “Solid matrices for surface enhanced Raman spectroscopy”; and U.S. Pat. No. 6,623,977 entitled “Materials for surface-enhanced Raman spectroscopy, and SER sensor and method for preparing same”, the disclosures of which are incorporated herein by reference.